He showed his skill as a tactician during the election campaign.
That man is just a schemer; I can't trust him.
策士 focuses on someone who is cunning at devising schemes, often with a slightly negative shade, while 参謀 is neutral and describes an official planning role.
Compound of 策 (plan, scheme) and 士 (person of a certain profession). The exact historical derivation is uncertain, but the word has been used since at least the Edo period to refer to a person skilled in devising strategies.